1. Field of the Invention
This invention relates to a foamed sheet of a base resin containing a polylactic acid resin as its major component, to a foam molding of such a polylactic acid resin-containing base resin and to a method of preparing a foam molding by thermoforming such a foamed sheet.
2. Description of Prior Art
Foamed bodies of generally employed resins such as polyethylene, polypropylene and polystyrene resins are now used in various fields because of their good heat insulating properties, cushioning properties and lightness in weight. These foam moldings, which are generally stable, remain in our environment when disposed of and cause environmental problems.
To cope with this problem, many studies are being made on polymers which are decomposable by microorganisms. Among various biodegradable polymers, polylactic acid resins have actually used as, for example, surgical suture materials. Polylactic acid resins are promising because they are safe to human bodies, because they are decomposable, when left in the environment, by hydrolysis and by biological degradation and because their starting material, lactic acid, can be prepared with a high yield at a low cost by fermentation of biomass such as corn. In particular, environmentally friendly foamed bodies such as foamed sheets of a polylactic acid resin are now being developed.
For example, Japanese Unexamined Patent Publication No. JP-A-2002-322309 discloses a foamed sheet of a non-crystalline polylactic acid resin. While the sheet is obtainable with ease by extrusion, the heat resistance thereof is poor. Japanese Unexamined Patent Publications Nos. JP-A-2000-136259 and JP-A-2002-3709 disclose a foamed sheet of a crystalline polylactic acid resin. While the crystalline polylactic acid resin has good heat resistance, it is difficult to obtain a foamed sheet because of poor moldability and poor foamability. Moreover, the foamed sheet obtained has poor thermofoamability because of a high apparent density, non-uniform cell shapes and a low closed cell content.